Molecular Playground/PPCA

A CBI Molecule being studied in the University of Massachusetts Amherst Chemistry-Biology Interface Program at UMass Amherst and on display at the Molecular Playground.

Human protective protein/cathepsin A (PPCA) is a lysosomal serine carboxypeptidase that has a chaperoning function towards two other lysosomal enzymes, lysosomal neuraminidase (Neu1) and beta-galactosidase (Glb1). Deficiency of this enzyme causes the autosomal recessive disease galactosialidosis for which there is no known treatment. Due to the secondary deficiencies of Neu1 and Glb1 in these patients the substrates of PPCA protease activity are not clearly known. PPCA is synthesized as a dimer (each monomer is 54kDa single chain) that undergoes processing in the lysosome to form a dimer of heterodimers, 32kDa and 20kDa subunits. The processing involves the removal of a 2kDa peptide called the excision peptide and consequent conformational changes to form the mature enzyme.



The dimer of PPCA is shown here in cartoon representation. The active site residues are in red with the disulphides in yellow. The catalytically competent active site is blocked by the 'blocking peptide' shown in cyan. The excision peptide shown in dark blue is positioned above the blocking peptide thus completely occluding the active site. The excision peptide is removed during processing which is assumed to result in local conformational changes that then remove the blocking peptide from occluding the active site.

Molecular Playground banner: Human lysosomal protease that exhibits chaperoning function.